The structure of organic electroluminescent devices (OLEDs) in which organic semiconductors are employed as functional materials is described, for example, in U.S. Pat. No. 4,539,507, U.S. Pat. No. 5,151,629, EP 0676461 and WO 98/27136. In recent years, the emitting materials employed are increasingly organometallic complexes which exhibit phosphorescence instead of fluorescence (M. A. Baldo et al., Appl. Phys. Lett. 1999, 75, 4-6). For quantum-mechanical reasons, an up to four-fold increase in energy and power efficiency is possible using organometallic compounds as phosphorescence emitters. In general, however, there is still a need for improvement in OLEDs which exhibit triplet emission. Thus, the physical properties of phosphorescent OLEDs are still inadequate with respect to efficiency, operating voltage and lifetime for use of triplet emitters in high-quality and long-lived electroluminescent devices. This applies, in particular, to OLEDs which emit in the relatively short-wave range, i.e. green and in particular blue. Thus, no blue-emitting triplet emitters which meet the technical requirements for industrial use are known to date.
In accordance with the prior art, the triplet emitters employed in phosphorescent OLEDs are, in particular, iridium complexes. An improvement in these OLEDs has been achieved by employing metal complexes with polypodal ligands or cryptates, as a consequence of which the complexes have higher thermal stability, which results in a longer lifetime of the OLEDs (WO 04/081017, WO 05/113563, WO 06/008069). However, these complexes are not suitable for blue emission, in particular for saturated deep-blue emission.